One class of conventional continuously variable transmissions (or “CVT's”) has two tapered-faced pulleys interconnected with a belt of essentially fixed length. The sheaves of each pulley are able, under control, to move axially. The pulley on one shaft is connected to the crankshaft of the engine. The system including the pulley, and its ancillary parts, that is connected to the engine is called the driving, driver, or primary clutch. The other pulley is connected through a linkage to the drive train of a vehicle. This other pulley and its related parts, is called the driven or secondary clutch. Of necessity, when the sheaves of either pulley are close together, the associated belt must be located at a relatively large radius (distant from the axis of rotation of the pulley) and when the sheaves of a pulley are far apart, the associated belt must be located at a relatively small radius (close to the axis of rotation of the pulley).
Typically, because of the essentially fixed length of the belt, when the sheaves of one pulley are far apart, then the sheaves of the other pulley must be close together. Larger shift ratios, characteristic of slower vehicle speeds, occur when the sheaves of the primary pulley are far apart and the sheaves of the secondary pulley are close together. With this configuration, the rotational speed of the primary pulley is greater than the rotational speed of the secondary pulley. Smaller shift ratios, characteristic of higher vehicle speed, occur when the sheaves of the primary pulley are close together and the sheaves of the secondary pulley are far apart. With this configuration, the rotational speed of the primary pulley is less than the rotational speed of the secondary pulley.
Ordinarily, the primary clutch has a compression spring, or the like, tending to push the sheaves apart such that, at rest, the sheaves of the primary pulley have opened to allow the belt to lie close to the pulley's rotational axis, effecting a large shift ratio. Such a belt position at rest results in the engine having a desirable minimal load at the start of driving. The force produced by this spring increases as the sheaves of the primary pulley get closer together (lower shift ratios) and further compress the spring. Other parts of the primary clutch include a set of pivoting flyweights on the primary clutch pushing on a roller, or the like, linked such that the sheave spacing, and thus shift ratio, is responsive to speed and torque needs of the secondary clutch.
In the known CVT systems, the net result of the spring and flyweights of the primary clutch provide some beneficial results. Specifically, these CVT systems yield a primary pulley belt side force that is sufficient to allow the engine to start and promptly get up to approximately a rotational speed where the engine can deliver maximum power to its shaft. These CVT systems also yield a belt side force that increases with increasing vehicle speed (decreasing shift ratio) to a peak. Another benefit of these CVT systems is that they provide a belt side force that decreases with increasing vehicle speed.
An undesirable result of these CVT systems is a tendency to lose power because of belt slippage due to insufficient belt side force while the vehicle is accelerating to near maximum speed. A desirable result of the just described belt side force is a tendency for the system to increase the shift ratio (deliver more torque) when the vehicle slows down.
The typical role of the engine is to start, to accelerate promptly to a high rotational speed where the engine can deliver approximately its maximum power, and to remain at that high speed delivering approximately a constant amount of power. Power, in this context, is the product of torque and rotational velocity (expressed as P=τ×ω, where P is power, τ is torque, and ω is rotation velocity). The role of the CVT is to apportion the power delivered by the engine into a torque and speed portion depending on the vehicle's speed. When the vehicle is moving slowly, the CVT has a high shift ratio, and the torque factor is relatively large. When the vehicle is moving rapidly, the CVT has a smaller shift ratio, and the torque factor is smaller.
It is known to use a clutch having a plurality of flyweights pivotally mounted on a single rotatable member, with the flyweights arranged to move radially outward with increasing rotational velocity of the shaft. For additional details, refer to U.S. Pat. No. 3,727,478 to Erickson.
U.S. Pat. No. 5,529,544 to Berto is another example of a system having a plurality of flyweights pivotally mounted on a single rotatable member. Specifically, this patent teaches a clutch having speed responsive means consisting of two sets of flyweights having different size, shape and/or weights. One of the first set of flyweights and one of the second set of flyweights are positioned side-by-side and have the same axis of rotation. The first and second sets of flyweights operate simultaneously by exerting an initial displacement force against the moveable sheave. Then, at a predetermined rotational speed of the drive clutch or predetermined position of the second set of flyweights, the flyweights of the second set are prevented from exerting force on or further displacing the moveable sheave. Thus, for rotational speeds greater than the preselected rotational speed, the first set of flyweights act alone in displacing the moveable sheave.
while the clutch disclosed in Berto yields some benefits, it still suffers from significant disadvantages. For example, due to the side-by-side arrangement of the first and second set of flyweights, the mass of the flyweights is limited due to space constraints and structural integrity constraints. Because of the limited mass of the flyweights, this arrangement cannot provide sufficient force necessary to perform the function of today's high performance engines.